Evidence.
Without the vehicle, how do you prove transport?
If there was a speedy trial, as mandated by the Constitution, they wouldn’t need to hold property for evidence.
Of course, if the defendant’s lawyers keep delaying the trial, then the “innocent” property owners are SOL.
But how innocent are you if you let drug dealers use your car or home? I guess it can happen, though.
That would be temporary, at best. Until the end of the trial.
But photos and direct testimony and all sorts of other ways prove transport. It’s not like they march a jury out and look at a vehicle.